1. Field of the Invention
The present invention relates to an image processing apparatus, such as a color copying machine and a color FAX, for processing color image signals to record a color image using recording materials (such as ink toners) of plural colors.
2. Related Background Art
When color recording is performed by the additive color mixture based on image signals obtained through color separation, generally black lines and characters, in particular, are recorded by superposing recording materials of three colors, i.e., Y, M and C, or four colors, i.e., Y, M, C and K.
However, the superposition of recording materials of those three or four colors raises the following problems:    (1) The resulting color by the superposition does not look perfectly black,    (2) Difficulties in high-accurate superposition causes a color shear or offset, which also makes the resulting color not look black, and    (3) In recording based on the pseudo-half-tone processing, the recording cannot be made by arraying dots in a continuous linear line. Thus, an image cannot be represented with high resolution. Meanwhile, it has been previously proposed in U.S. patent application Ser. No. 378,488, now U.S. Pat. No. 5,029,227, a method of representing a monochromatic image by discriminating and separating portions of lines and characters in an image from other image portions of half tone, and recording only the former portions with high resolution, thereby allowing both gradation and resolution of the image to stand together in representation.
Although this method can solve the above problem (3), it has a shortcoming to require independent hardwares for different colors when applied to processing of a color image. Further, a method of discriminating a black portion from color signals and judging a region which coincides with a character portion extracted from the component of one color, e.g., the G component, as a black character region still accompanies the problems that the black portion is difficult to discriminate inexpensively with high accuracy, and that the judged black region is not always precisely coincident with the actual black region. In addition, the so-called black signal produced by masking or UCR cannot be employed for this type processing because that black signal is adapted to correct only black portions which can be represented by using C, M and Y, and thus has not a sufficient level for line portions of black characters, in particular. To solve the above problems, U.S. patent application Ser. No. 173,654, U.S. Pat. No. 4,958,219, U.S. patent application Ser. No. 327,098, now U.S. Pat. No. 5,047,844, U.S. patent application Ser. No. 367,673, now U.S. Pat. No. 5,031,034, and U.S. patent application Ser. No. 519,500, now U.S. Pat. No. 5,113,252, have been proposed, but there still remains room for improvement.
Furthermore, it is conventional in image processing apparatus that a color image is subjected to color separation into three colors, i.e., R, G and B, and C, M, Y and K (black) signals are then produced as record colors, followed by the pseudo-half-tone processing for each of the four colors. As a pseudo-half-tone processing technique, while there is known for many years a dither method which can be constituted inexpensively, a data storage type technique represented by an error dispersing method has become more popular in recent years which carries out error correction through re-quantization for enabling resolution and gradation to be reproduced in a substantially compatible manner.
In the above pseudo-half-tone processing technique of data storage type (i.e., the conditioned dither method), however, re-quantized dots are not directly determined from input data of one point. For the reason, when input image data obtained through color separation into four colors are processed independently, the re-quantized dots in four colors may locally overlap with each other. Accordingly, in the case that recording is made using the electrostatic process based on the re-quantized data, for example, toners of four colors locally overlap with each other to form a projection with the height of several 100 microns on the sheet surface of transfer paper, resulting in drawbacks of lowering a fixing property and degrading an image quality.
In the case of recording an image by an ink jet technique, for example, a large amount of ink is jetted into a local area. This results in similar drawbacks that an ink smear occurs on the rear side of recording paper and a longer period of time is required for drying.
In addition, when deposit positions of recording materials of different colors are not perfectly coincident with each other for each pixel by virtue of limitations in mechanical accuracy, a color shear is caused and color reproducibility is badly deteriorated.